Abstract

The trace element chemistry of anoxic sediments and sedimentary rocks has previously been shown to correlate with the chemistry of the ocean (Algeo., 2004). Recent studies have used the trace
element content of ancient sedimentary pyrite to track first order changes in ocean chemistry through
geological time and display good parallels with traditional whole-rock methods (Large et al. 2014,
2015). However, no evidence for the possible diagenetic effects on the preservation of the authigenic
seawater signal has been presented for this or other methods. Without the preservation of coeval
seawater to offer a direct comparison, the possibility of diagenetic overprints requires that
sedimentary pyrite analysis and other methods should only be considered semi-quantitative. The
recent IODP Exp. 347 "Baltic Sea Paleoenvironment" offered an opportunity to bridge this current
knowledge gap by evaluating the formation and chemistry of recent pyrite in a well-constrained
context. The Baltic Sea Basin has experienced a varied history in the short time since its formation
and has recorded a diverse series of environmental changes. Preserved in the high-resolution
stratigraphy recovered during Exp. 347 are several sedimentary units which represent fluxes between
a geographically and geochemically restricted water mass, freshwater, and brackish seawater, as well
as the development of anoxic water column conditions. Here we present sedimentological results
coupled with high-resolution geochemical profiles through the stratigraphy in order to build an
integrated understanding of the variations of chemical partitioning within the sediments and to
constrain how such observations may relate to changes in the seawater chemistry.

Analysis of the uppermost section of the sapropel shows that it captures the sediment-water interface
and the uppermost seawater saturated horizon, where pyrite forms. We observe that within the upper
~2m of the sediment there are systematic variations in both sulphide minerals and their geochemistry.
These mineralogical and chemical differences appear to be correlated with redox horizons, the
interface between the break down of organic matter, and changes in the pore water content and
biologically-induced SO4-CH4-H2S fronts (Fig. 1). Here, the dominant sulphide species vary between
pyrite and pyrite-greigite. Below this horizon, pyrite dominates the sulphide mineral assemblage and
its chemistry become relatively constant. We suggest that this zone has captured the dynamics,
pathways and metal partitioning during the formation of syn-sedimentary pyrite and on a short time
scale, less than that of residence times of many elements (i.e 100s to 1000s of years).

We have applied the pyrite chemistry technique to sections deeper in the stratigraphy and other subbasins
within the Baltic Sea in order to investigate horizons that may archive redox or basin chemical
changes. Our data shows enrichments in specific trace metals in pyrite in horizons that mark critical
changes in the development of the Baltic Sea and those that have independently been identified as
euxinic and anoxic (Hardisty et al., in prep.). Here we discuss the dynamics and pathways of initial
pyrite growth and present a series of examples where we evaluate the use of pyrite chemistry as an
archive of redox changes in the Baltic Sea.